1. Field of the Invention
The present invention relates to a voltage level shifting circuit which outputs a signal outputted from a circuit which operates on low power supply voltage to a circuit which operates on high power supply voltage. Especially, the present invention relates to a voltage level shifting circuit which is suitable for a drive circuit for driving a LCD panel and a drive circuit for driving a fluorescence display tube.
2. Description of the Related Art
For example, the U.S. Pat. No. 5,113,097 (Lee) exists as technology relevant to the present invention. In FIG. 4 of Lee, a level shifting circuit which changes an incoming signal Vin having logic levels of 0v (Vss1) and 15v (+VDD) into an output signal Vout having logic levels of −5 v (−Vss2) and 15v (+VDD) is described. That is, changing the signal which has relatively small logic amplitude into the signal which has relatively large logic amplitude is described in Lee.
Referring to FIG. 4 of Lee, the PMOS transistor 206 and the PMOS transistor 208, which have gate electrode receiving the incoming signal Vin which has small logic amplitude, are connected between the power supply +VDD and the power supply −Vss2. The difference between the potential level of the power supply +VDD and the potential level of the power supply Vss1 is 15v. On the other hand, the difference between the potential level of the power supply +VDD and the potential level of the power supply −Vss2 is 20v, and this value is larger than the difference between the potential level of the power supply +VDD and the potential level of the power supply Vss1. Therefore, it is required that withstand voltages of the PMOS transistor 206 and the PMOS transistor 208 are higher than those of MOS transistors which constitute the inverter 404 and the inverter 202. (In order to simplify an explanation, in subsequent explanation, a transistor having lower withstand voltage is referred to as a low withstand voltage transistor, and a transistor having higher withstand voltage is referred to as a high withstand voltage transistor.) By the same reason, the PMOS transistor 408, the NMOS transistor 210, the NMOS transistor 212, and the NMOS transistor 410 comprise the high withstand voltage transistor, and each transistor which constitutes the inverter 404 and the inverter 202 comprise the low withstand voltage transistor.
When gate to source voltages are set into the same value, generally it is known that the current supply capability of the high withstand voltage transistor is low as compared with the current supply capability of the low withstand voltage transistor. FIG. 1 is a figure showing the difference of the current supply capability of the high withstand voltage transistor and the current supply capability of the low withstand voltage transistor. In FIG. 1, an NMOS transistor whose gate to source voltage Vgs is set at 5v is explained as an example. L indicates the gate length of the transistor and W indicates the gate width of the transistor. IDS indicates drain to source current and VDS indicates drain to source voltage. In FIG. 1, when drain to source voltage is set at 1.0v, the low withstand voltage transistor can flow 3.0 mA current and the high withstand voltage transistor can flow only 500 μA current are shown as an example. This means that the size (dimension of a transistor) of the high withstand voltage transistor needs to be set up by about 6 times the size of the low withstand voltage transistor in order to obtain the current supply capability equivalent to the low withstand voltage transistor. The current supply capability relates also to an operation speed of a transistor. This means also that the size (dimension of a transistor) of the high withstand voltage transistor needs to be set up by about 6 times the size of the low withstand voltage transistor in order to obtain the operation speed equivalent to the low withstand voltage transistor.
In the drive circuit for driving the fluorescence display tube and the drive circuit for driving the LCD panel, level shifting circuits corresponding to a number of the display element are required. The number is several 100 circuits from some dozens of circuits. Therefore, when the voltage level shifting circuit having above described structure is simply applied to the above mentioned drive circuit, there is a possibility that an area on a chip for forming the drive circuit may increase in order to increase the operation speed of the drive circuit more. Or in order to reduce the size of the area on the chip for forming the drive circuit more, or in order to maintain the area on the chip, there is a possibility that it may be difficult to make the operation speed of the drive circuit increase. Therefore, there has been a demand for a voltage level shifting circuit which has an improved operation speed and an improved circuit area for forming a circuit.